A conventional protection device for electric appliance uses fuses which are broken when the fuses are overheated due to overload. After the fuses are broken, the circuit is cut off to prevent the electric appliance from being burned. In the modern design of the electric appliance, several circuits are involved and each circuit has a switch or protection device so as to achieve multiple layers of protection.
As shown in FIG. 1, a latest protection device includes a bi-metallic plate 201 which has a first end fixed to a first terminal 202 and a second end is a free end which has a first contact point 203 connected to an underside thereof. A second contact point 205 is fixed on a second terminal 204 and when the first and second contact points 203, 205 are in contact with each other, the circuit is in “ON” status. When the circuit is overloaded, as shown in FIG. 2, the high temperature makes the bi-metallic plate 201 bend upward so that the first and second contact points 203, 205 are separated and the circuit is in “OFF” status. By this way, the appliance is protected from being burned. However, it is a difficult task to ensure and to manufacture each bi-metallic plate having the same physical characteristics. Some of the bi-metallic plates do not bend at desired temperature and the range of the temperature is too wide to precisely set the bi-metallic plates to have the same characteristics. Furthermore, some bi-metallic plates are not so sensitive to the temperature and do not react as desired. Besides, when the bi-metallic plate bends less than as desired, the small gap between the two contact points might generate sparks to burn the whole appliance.
The present invention intends to provide a dual protection device that includes a bi-metallic plate and a contact plate which is connected to a carrier of one of the contact point by low melting metal which melts at desired temperature to cut off the circuit.